Method and apparatus for a signalling message transport mechanism

ABSTRACT

A method of transmitting signalling information in a telecommunications network between peer user/application parts. Signalling information is transferred from a first user/application part to a Message Transfer Part (MTP) level 3, the information including a destination signalling point identifier identifying the signalling point at which the peer user/application part is located. The MTP level 3 determines from the destination signalling point identifier, a destination address suitable for conveying the signalling information to the destination signalling point. In the event that the destination address is an Internet Protocol (IP) address and Port number, the signalling information and the determined IP address and port number are transferred to an IP part for transmission over an IP network to the destination signalling point.

FIELD OF THE INVENTION

The present invention relates to a transport mechanism for signallingmessages in a communications network and more particularly, though notnecessarily, to a transport mechanism for signalling messages which aretraditionally carried by a Signalling System No. 7 based network.

BACKGROUND TO THE INVENTION

In a telecommunications system, signalling equipment and signallingchannels are required for the exchange of information between systemnodes. In particular, this internode signalling informs traffic channelswitches of what is to be performed when a telephone or data call is tobe set up or released in so-called “circuit-switched” connections.Signalling is also used to query centrally located databases, e.g. toobtain routing information for 800 numbers and to determine the locationof a subscriber in a Public Land Mobile Network (PLMN).

Modern telecommunications systems now largely make use of Common ChannelSignalling (CCS) whereby signalling information is transmitted on one ormore dedicated signalling channels, distinct from the channels used tocarry actual user information (e.g. voice or data). An important featureof CCS is that the same signalling system may support services in avariety of existing telecommunications protocols, e.g. Public SwitchedTelephone Network (PSTN), Integrated Services Digital Network (ISDN),and Public Land Mobile Networks (PLMN), as well as proposed futureprotocols such as B-ISDN, enhancing greatly the interoperability ofnetworks supporting different protocols.

Currently, the predominant CCS is known as Signalling System Number 7(SS7), defined in the ITU-T (International TelecommunicationsUnion—Telecommunications section) recommendations starting with Q.700,and by ANSI (American National Standards Institute) in recommendationsT1.111. SS7 is a packet switched system having multiple signalling linksof one time slot in a Time Division Multiple Access (TDMA) E.1 or T.1transmission format (the other time slots being available for userdata). Individual signalling message packets (datagrams) are associatedwith respective individual telephone calls. As only a relatively smallamount of signalling information is associated with a single telephonecall, a single SS7 channel is able to handle all signalling between twonetwork nodes (termed “signalling points”) for several thousands ofcalls.

As already alluded to above, SS7 is able to provide a signalling messagetransport mechanism for a number of different applications. FIG. 1illustrates an SS7 protocol stack of which the lowest level, MessageTransfer Part (MTP) level 1, defines the physical, electrical, andfunctional characteristics of a digital signalling link. MTP level 1 hasa number of different possible forms including the European standard E.1(2048 kb/s and 32 64 kb/s channels). MTP level 2 takes care of theaccurate end-to-end transmission of messages across a chosen signallinglink whilst MTP level 3 handles the routing of signalling messagesbetween neighbouring signalling links based upon information receivedfrom higher SS7 levels concerning the final destination of a signallingmessage. MTP level 3 handles inter alia re-routing of messages away fromfailed or congested signalling links.

Above the MTP levels, SS7 comprises an ISDN User Part (ISUP) whichdefines the protocol and procedures involved in setting-up, controlling,and tearing-down circuit switched connections which carry voice and dataover the Public Switched Telephone Network (PSTN). ISUP is not only usedin ISDN networks, but is also employed in non-ISDN networks. A TelephoneUser Part (TUP) supports basic call processing for analogue calls and isused for example in China.

With the development of advanced network functions such as freephone(800 numbers), call forwarding, mobile roaming, etc, it has beennecessary to add to SS7 a number of additional levels to support thesefunctions. In particular SS7 is now provided with a SignallingConnection Control Part (SCCP) which handles the routing of signallingmessages to and from the numerous applications which use the SS7transport mechanism. SCCP also provides for the transformation of aGlobal Title (e.g. a dialled 800 number) into a destination signallingpoint (in Europe a signalling point is defined by a combination of aNetwork Indicator and a Signalling Point Code).

Located above the SCCP is a Transaction Capabilities Application Part(TCAP) which is a protocol for dealing with the exchange of informationbetween signalling points related to database queries. For example, aswitch within a network may use TCAP to determine the B-numberassociated with a dialled 800 number. This may require the sending of asuitable TCAP message to a database associated with the 800 number, withthe SCCP performing the identification of signalling point at which thedatabase is located.

The applications which make use of TCAP, and by extension SCCP, aremany. For example the Mobile Application Part (MAP) is used in PLMNs totransfer information between Mobile Switching Centres (MSCs), HomeLocation Registers (HLRs), and Visitor Location Registers (VLRs), whilstthe Intelligent Network Application Part (INAP) is used to transferinformation between intelligent network nodes and telephone switches.

It will be appreciated that SS7 has evolved into a complex and hencecomputationally intensive set of protocols. There therefore exists, adesire to both reduce this complexity and also to reduce the high costsof signalling networks (both in terms of infrastructure and ofmaintenance). Telecommunications network designers have turned for ananswer to the field of data networks and in particular to InternetProtocol (IP) networks where high demand has lead to relatively low costand technically advanced solutions.

Several proposals have been put forward to employ IP networks intelecommunications signalling. For example, it has been proposed toencapsulate TCAP messages into IP datagrams for transmission over an IPnetwork, with a mapping being made between Global Titles (and/ordestination signalling points) and IP addresses for the purpose ofrouting the datagrams.

SUMMARY OF THE PRESENT INVENTION

It has been recognised by the inventors of the present invention, thatwhilst the introduction of IP transport mechanisms at higher levels inthe SS7 protocol stack, e.g. directly beneath the TCAP, results incertain advantages, it results in a piecemeal solution where eachapplication or user part, or at least small groupings of application anduser parts, require individual IP address mapping tables. In the case ofTCAP over IP (or SCCP over IP), non-TCAP (or SCCP) users such as ISUPand TUP do not have access to the IP network.

It is an object of the present invention to overcome or at leastmitigate the above noted disadvantage. This and other objects areachieved at least in part by replacing MTP level 2 with the IP basedprotocol levels.

According to a first aspect of the present invention there is provided amethod of transmitting signalling information in a telecommunicationsnetwork between peer user/application parts, the method comprising:

-   -   transferring signalling information from a first        user/application part to a Message Transfer Part (MTP) level 3,        the information including a destination signalling point        identifier identifying the signalling point at which the peer        user/application part is located:    -   determining at the MTP level 3, from said destination signalling        point identifier, a destination address suitable for conveying        the signalling information to the destination signalling point        or to an intermediate signalling point en route to the        destination signalling point; and    -   in the event that said destination address is an Internet        Protocol (IP) address and port number, transferring the        signalling information and the determined IP address and port        number to an IP part for transmission over an IP network to the        destination or intermediate signalling point.

Embodiments of the present invention enable all user/application partssharing a common MTP level 3 (e.g. ISUP, TUP. TCAP/SCCP, etc) to alsoshare a common table mapping destination signalling point identifiers toIP addresses and port numbers. Thus, changes in IP routing information(and/or destination signalling point changes) need be reflected only inthe single common table, and there is no need to update multipletranslation tables.

Preferably, the method of the present invention comprises:

-   -   receiving the signalling information transmitted over the IP        network at the signalling point identified by said IP address        and port number; and    -   passing the signalling information to an MTP level 3 and        determining whether or not the signalling point is the        destination signalling point on the basis of said destination        signalling point identifier included in the signalling        information.

Preferably, the method comprises transferring the signalling informationto an MTP level 2 in the event that the destination address determinedby the MTP level 3 is a signalling link, and transmitting theinformation to the destination signalling point, or to an intermediatesignalling point, over the signalling link.

In the event that the receiving signalling point is the destinationsignalling point, the signalling information is passed to the peeruser/application part. If the receiving signalling point is not thedestination signalling point, then the MTP level 3 determines a furtherdestination address, on the basis of the destination signalling pointidentifier, suitable for conveying the signalling information to thedestination signalling point or to another intermediate signallingpoint. The signalling data may be transmitted to the destinationsignalling point, or to the next intermediate signalling point, via anIP network as already described or over an alternative network such asan SS7 network (MTP level 2).

Preferably, the method comprises providing a look-up table at theoriginating signalling point which maps signalling point identifiers toIP addresses and port numbers or to signalling links.

Preferably, the method comprises providing an adaptation level betweenthe MTP level 3 and the IP part, at the originating, intermediate, anddestination signalling points. The adaptation layer “listens” to apredetermined port number to receive and process incoming TCPconnections or UDP packets, and provides an interface between the MTPlevel 3 and the TCP/UDP levels.

The signalling point identifier may comprise a Network Indicator (NI)and a Signalling Point Code (SPC), where the NI identifies a network andthe SPC identifies a signalling point within that network.Alternatively, the signalling point identifier may have a NetworkIndicator plus “Network Identifier—Network Cluster—Network ClusterMember” format (according to the US standard) or some other such format.

According to a second aspect of the present invention there is providedapparatus for transmitting signalling information in atelecommunications network between peer user/application parts, theapparatus comprising:

-   -   first processing means implementing a Message Transfer Part        (MTP) level 3 for receiving signalling information from a first        user/application part, the information including a destination        signalling point identifier identifying the signalling point at        which the peer user/application part is located, the MTP level 3        determining from said destination signalling point identifier, a        destination address suitable for conveying the signalling        information to the destination signalling point or to an        intermediate signalling point en route to the destination        signalling point; and    -   second processing means implementing an IP part for transmitting        the signalling information and the determined IP address and        port number over an IP network to the destination or        intermediate signalling point, in the event that said        destination address is an IP address and port number.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and in order to showhow the same may be carried into effect reference will now be made, byway of example, to the accompanying drawings, in which:

FIG. 1 shows schematically the protocol levels in an SS7 protocol stack;

FIG. 2 shows schematically a telecommunications network;

FIG. 3 shows schematically the protocol levels in an IP based signallinginformation transport mechanism;

FIG. 4A is a flow diagram illustrating the process of transmittingsignalling messages from a signalling point of the network of FIG. 2;and

FIG. 4B is a flow diagram illustrating the process of receivingsignalling messages at a signalling point of the network of FIG. 2.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

A conventional SS7 protocol stack has already been described withreference to FIG. 1. An alternative to this SS7 transport mechanism willnow be described, firstly with reference to FIG. 2 which showsschematically a telecommunications network.

The telecommunications network comprises a first signalling point 1which, for the purpose of the present example, is a Mobile SwitchingCentre (MSC) of a PLMN. A second switching point 2 of the PLMN is aVisitor Location Register which maintains a record of mobile subscribersregistered with the MSC 1, their status (i.e. on or off), and theircurrent location (i.e. cell). In the event that a call is initiated to asubscriber for whom the MSC 1 is the “home” exchange, before the callcan be set up it is necessary for the MSC 1 to exchange certainsignalling information with the VLR 2. The form of this information willnot be described here in detail, other than to note that it is theMessage Application Parts (MAPs) at the MSC 1 and VLR 1 which are theusers of the exchanged information, with the MAPs communicating viarespective TCAPs and SCCPs.

As will be described below, MAP messages are transported between the MSC1 and the VLR 2 using an IP network indicated generally by referencenumeral 3 in FIG. 2. The IP network may comprise a number of Ethernetlinks or the like, with intermediate IP routers. In addition to the MSC1 and the VLR 2, other signalling points may be connected to the same IPnetwork 3 although these are not shown in the Figure.

The role of the MTP level 3 in selecting a signalling link forsignalling information in an SS7 network has been described above asbeing to select a signalling link on the basis of routing informationreceived, in the case of MAP messages, from the SCCP. In Europe, thisrouting information is typically a signalling point identifiercomprising a Network Indicator (NI), a destination Signalling Point Code(SPC) and a Signalling Link Selection (SLS). The MTP level 3 makes useof a look-up (or routing) table to perform the mapping between thesignalling point identifiers and signalling links. The look-up table isillustrated in FIG. 2 by a database 4.

For the purposes of transporting signalling information via theavailable IP network 3, the look-up table used by the MTP level 3 ismodified to replace signalling links with IP addresses and port numbers,where respective signalling points are connected to the IP network 3.When an IP address and port number are available for a destinationsignalling point, the MTP level 3 routes the signalling information(plus address) to a set of protocol levels which effectively replace theMTP levels 1 and 2 of the conventional SS7 protocol stack.

These replacement levels include a TCP/UDP layer which is responsiblefor the encapsulation of the signalling information into datagrams andfor ensuring the error free transmission and of the datagrams.Encapsulated datagrams are then passed to an IP level which isresponsible for routing the datagrams over the IP network 3 on the basisof the associated IP addresses and port numbers. Beneath the IP level isthe physical level which may be Ethernet, X.25, or the like.

An adaptation level is disposed between the MTP level 3 and the TCP/UDPlevels and acts as an interface for sending MTP messages to the TCP/UDPlevel and vice versa. The adaptation layer also listens to a predefinedport number to receive and process incoming TCP connections and UDPpackets, as well as monitoring the availability of MTP 3 levels atremote signalling points. In the event that an event occurs in the IPnetwork (e.g. the route to a remote signalling point goes down, the TCPconnection is dropped, or a new TCP connection has been received), thisis signalled to the MTP 3 level by the adaptation layer.

Upon arrival at the signalling point associated with the IP address,i.e. VLR 2, the signalling messages are decapsulated by the TCP or UDPlevel and are passed to an adaptation layer which is assigned the portnumber conveyed with the received signalling messages. The adaptationlayer passes the messages to an MTP level 3, which confirms that thereceiving signalling point is indeed the destination for the messages onthe basis of the destination signalling point identifier whichaccompanies the message. The MTP level 3 then passes the messages to thepeer MAP/TCAP via the SCCP level.

In some cases, a destination signalling point for a signalling messagemay not be connected to the IP network 3, such that it is necessary toroute signalling messages via an intermediate signalling point. This isillustrated in FIG. 2, where signalling point 2 is now considered as anintermediate signalling point whilst signalling point 5 is thedestination signalling point.

In this case, the IP address and port number returned by the database 4at the MSC 1 identify the intermediate signalling point 2. Upon receiptof a message at the intermediate signalling point 2, the message isdecapsulated at the TCP or UDP level and is passed to the MTP level 3via the adaptation layer. From the destination signalling pointidentifier contained in the message, the MTP level 3 determines that theintermediate signalling point 2 is not the destination signalling pointand accordingly selects a destination address appropriate to the actualdestination signalling point 5, using a database 4 associated with thereceiving signalling point 2.

Assuming that a second IP network 6 connects the intermediate signallingpoint 2 to the actual destination signalling point 5, then the databasequery returns an IP address and port number associated with thedestination signalling point 5.

It may alternatively be the case that a destination/intermediatesignalling point is not connected to the originating signalling pointvia the IP network 3 (either directly or indirectly via an intermediatesignalling point), but rather by a conventional SS7 network. In thiscase, when the MTP level 3 at the originating signalling point queriesthe database 4 with the destination signalling point identifier, asignalling link is returned (rather than an IP address and port number).The MTP level 3 then directs the signalling information to the MTP level2 (illustrated in FIG. 3), for transport over the SS7 network.Similarly, an intermediate signalling point may have the option ofdirecting signalling information over an IP network or an SS7 network,depending upon availability.

FIG. 4A is a flow diagram illustrating the process of transmitting asignalling message over an IP network to a signalling point, whilst theflow diagram of FIG. 4B illustrates the process of receiving asignalling message at a signalling point.

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above described embodiment withoutdeparting from the scope of the present invention. For example, whilstthe above description has been concerned with the transmission of MAPmessages, the invention is also applicable to the transmission of othersignalling messages which make use the MTP level 3 e.g. ISUP, TUP, etc.

1. A method of transmitting signalling information in atelecommunications network between peer user/application parts, themethod comprising: transferring signalling information from a firstuser/application part to a Message Transfer Part (MTP) level 3, theinformation including a destination signalling point identifieridentifying the signalling point at which a peer user/application partis located; determining at the MTP level 3, from said destinationsignalling point identifier, a destination address suitable forconveying the signalling information to the destination signalling pointor to an intermediate signalling point en route to the destinationsignalling point; providing an adaptation layer between the MTP level 3and the IP part at the originating signalling point, as well as atintermediate and destination signalling points, the adaptation layerproviding an interface between the MTP level 3 and TCP/UDP levels andlistening to a predetermined port number to receive and process incomingTCP connections or UDP packets; and in the event that said destinationaddress is an Internet Protocol (IP) address and port numbertransferring, via the adaptation layer, the signalling information andthe determined IP address and port number to an IP part for transmissionover an IP network to the destination or intermediate signalling point.2. A method according to claim 1 and comprising transferring thesignalling information to an MTP level 2 in the event that thedestination address determined by the MTP level 3 is a signalling link,and transmitting the information to the destination signalling point, orto an intermediate signalling point, over the signalling link.
 3. Amethod according to claim 1 and comprising: receiving the signallinginformation transmitted over the IP network at the signalling pointidentified by said IP address and port number; and passing thesignalling information via the adaptation layer to the MTP level 3 anddetermining whether or not the signalling point is the destinationsignalling point on the basis of said destination signalling pointidentifier included in the signalling information.
 4. A method accordingto claim 3 and comprising passing the signalling information to the peeruser/application part in the event that the receiving signalling pointis the destination signalling point.
 5. A method according to claim 3and comprising determining at the MTP level 3 a further destinationaddress, on the basis of the destination signalling point identifier,suitable for conveying the signalling information to the destinationsignalling point or to another intermediate signalling point, if thereceiving signalling point is not the destination signalling point.
 6. Amethod according to claim 1 and comprising providing a look-up table ata transmitting signalling point, which table maps signalling pointidentifiers to IP addresses and port numbers or to signalling links. 7.A method according to claim 1, wherein the adaptation layer monitors theavailability of MTP 3 levels at remote signalling points and reportsnetwork events to the associated MTP 3 level.
 8. A method according toclaim 1, wherein the signaling point identifier comprises a NetworkIndicator (NI) and a Signalling Point Code (SPC), where the NIidentifies a network and the SPC identifies a signalling point withinthat network.
 9. A method according to claim 1, wherein the signallingpoint identifier has a Network Indicator plus “NetworkIdentifier—Network Cluster—Network Cluster Member” format.
 10. Apparatusfor transmitting signalling information in a telecommunications networkbetween peer user/application parts, the apparatus comprising: firstprocessing means implementing a Message Transfer Part (MTP) level 3 forreceiving signalling information from a first user/application part, theinformation including a destination signalling point identifieridentifying the signalling point at which a peer user/application partis located, the MTP level 3 determining from said destination signallingpoint identifier, a destination address suitable for conveying thesignalling information to the destination signalling point or to anintermediate signalling point en route to the destination signallingpoint; second processing means implementing an IP part for transmittingthe signalling information and the determined IP address and port numberover an IP network to the destination or intermediate signalling point,in the event that said destination address is an IP address and portnumber; and an adaptation layer between the MTP level 3 and the IP partat the originating, intermediate and destination signaling points, theadaptation layer providing an interface between the MTP level 3 andTransmission Control Protocol/User Datagram Protocol (TCP/UDP) levelsand listening to a predetermined port number to receive and processincoming TCP connections or UDP packets.